A brief discussion on the "frequency response curve" in audio

What is a frequency response curve?

"Frequency response curve" decomposition: "Frequency" refers to "frequency", which is the same as "pitch" in sound performance; "response" can be regarded as the response of the speaker system (mechanical and electrical) to the conversion of "frequency" in the input electrical signal into sound energy. This response is received by the microphone and presented in the form of dB SPL logarithm after calculation by the test instrument. When many "frequency" response values ​​are connected together, it becomes a "curve" with peaks and valleys. This curve is called a frequency characteristic response curve, or frequency response curve for short.

Speakers and frequency response curve

Should the frequency response curve of an audio system or speaker product be flat? Many people debate this issue, and the focus of the debate is often that what sounds good is not necessarily flat, and what is flat is not necessarily good.

For example, if the curve of a speaker is more prominent near 80 Hz, it means that the speaker has too strong performance in the frequency band near 80 Hz, and if you play music, the bass will sound heavy. Or if the curve of a speaker is concave near 1000 Hz, it means that the speaker has weak performance in the frequency band near 1000 Hz, and the sound pressure output of the input signal near 1000 Hz is reduced, and the sound output is not the same as before.

The flatness of the frequency response curve actually tells you the difference in gain of this speaker or audio system for sound signals in different frequency bands. The flatter the curve is, the closer the gain of each frequency band of the speaker or audio system is to the same. However, the same gain of the speaker or system for each frequency band of the input signal does not equal good sound. Why? Because the same gain only expresses the same amount of amplification of the sound in each frequency band of the input signal. For example, a system has a gain of 30 decibels for each frequency in the full audio. If you make a sound of 1000 Hz and the sound pressure level is 80 decibels, the sound pressure level of the 1000 Hz sound emitted by the speaker is 80+30=110 decibels. If the sound pressure level of the 2000 Hz sound you make is 60 decibels, then the sound pressure level of the 2000 Hz sound played by the speaker is 90 decibels. Before the system amplification, the sound pressure level of the 1000 Hz sound and the 2000 Hz sound you emit differs by 20 dB. Then through this system with the same gain for each frequency band, the sound pressure level of the 1000 Hz sound and the 2000 Hz sound emitted by the speaker also differs by 20 dB, and the formation remains unchanged, haha. However, if the gain of your system for 1000 Hz is too large (protruding on the curve), not 30 dB but 40 dB, and the gain for 2000 Hz is too low (concave on the curve), not 30 dB but 20 dB. Then the 1000 Hz sound with a sound pressure level of 80 dB that you originally emitted will emit a sound of 120 dB after passing through the system, and the 2000 Hz sound with a sound pressure level of 60 dB that you originally emitted will emit a sound of 80 dB after passing through the system. The difference between the sound pressure levels of 1000 Hz and 2000 Hz before the system is 20 dB, and after the system, the difference between the sound pressure levels of 1000 Hz and 2000 Hz becomes 40 dB, which is no longer the original difference. The formation has changed, which is also a kind of distortion. Therefore, whether the frequency response curve is flat only represents whether the volume performance of a certain speaker or a certain system for the sound of each frequency band is roughly the same, and has nothing to do with the sound quality.

As for whether it sounds good or not, first of all, your system should have roughly the same gain for the input signal in each frequency band (that is, the curve should be as straight as possible), so that the proportion of the sound size of each frequency band in the original signal can be amplified and restored, at least the strong ones should be strong, and the ones that should not be strong should be weak. It can truly reflect the strength of the sound, which is a good foundation. If you want to sound good, it is more important to work on the sound quality. If the sound quality is bad, no matter how good the system is, it will still sound bad. If you don't believe it, get a good speaker, use a few dozen MP3s to input it into the mixer, and turn the mixer input gain to the top, play music downloaded from the Internet in MP3 format, and listen to the sound. The sound quality is something internal, not just a matter of the straightness of the curve. The straight curve only expresses the system's restoration of the volume. Then the restoration of the sound quality is probably an idealized thing. For example, it is almost impossible to completely restore the texture of a piano piece recorded with a Steinway piano and a DPA microphone through the sound system. It's like listening to someone playing the violin next to you, and listening to the same person playing the violin next to the speakers. Even if you use the best speakers, there will always be a difference in sound quality and sound field restoration. These restorations are not something that can be expressed by curves. The quality of the sound has a lot to do with the materials you use, your craftsmanship, the designer's skills and artistic accomplishment. Can a work of art carved by a master with white jade look the same as something cast out of plaster by a street craftsman?

On the other hand, the curve is straight. The curve is straight, which means that the system or device has a high degree of restoration of the volume strength of each frequency band in the input signal. As a sound system, this is only a basic indicator, but it is also a very important indicator. For example, a sound system with good volume restoration, the input sound source signal itself has a harmonious volume ratio of each part (such as a music master-level work recorded by a recording master, like some audiophile heavenly discs). It will naturally feel harmonious when restored through the speakers. If the input signal is a song sung by a karaoke-level singer who can only shout and scream, the high, medium and low bass are not harmonious, and it will naturally be disharmonious when it comes out of a high-reduction sound system. However, a system with poor restoration, such as a frequency response curve that protrudes in the low frequency and is a little concave in the mid-high frequency, may make the bass that was not very strong become stronger, and the trumpet that should be strong become weaker, and the work that originally played the harmonious volume of each part may become disharmonious. However, if by chance the musician plays the bass too softly, or blows the trumpet too loudly, then two negatives equal a positive, and a work that originally had a discordant volume may sound more harmonious and pleasant using speakers with a low degree of restoration than using speakers with a higher degree of restoration.

In addition, for audio products, not only speakers, but also amplifiers, mixers and other peripheral devices have frequency response curves. According to industrial standards, these devices are required to have a flat frequency response curve without adjustment. The purpose is to require these devices to first maintain a faithful restoration of the volume strength of the signal characteristics as much as possible. If the equalizer you use has a high frequency response curve at 80 Hz and a low frequency response curve at 1000 Hz without adjustment and the faders are flat, would you still want it?

Headphones and frequency response curve

It is not easy to judge the quality of headphones by the frequency response curve.

The center of the earphone sound membrane is low frequency and the edge is high frequency.

The low-frequency end of the frequency response curve shows a downward trend. In order to obtain more low-frequency kinetic energy, the spherical design in the center of the earphone is to increase its surface area to obtain bass. The mid-frequency frequency response curve of the earphone is relatively flat because of the spiral texture on the surface of the sound membrane.

There is a large sawtooth on the frequency response curve of the high-frequency end of the earphone. This is because there is a soft ring on the edge of the sound membrane to increase the elasticity of the sound membrane. Therefore, the resonant frequency of the soft material decreases. After passing the soft ring and reaching the bonding edge, the material becomes hard, and the resonant frequency rises, forming a large sawtooth. Every earphone has this problem.

There are many small sawtooth on the frequency response curve of the high frequency end of the earphone, and there are burrs on the edge of the sound membrane bracket and the sound membrane. It is related to the manufacturing process of the earphone. If the bracket and the sound membrane are integrated, there will be no such problem.

Knowing the above situation, we should pay attention to the frequency response curve when choosing headphones. The low-end gain should be large, the high-frequency sawtooth should be small, and the mid-frequency should be flat.

Sound quality and frequency response curve

There are too many factors that affect sound quality.

First, let's take a look at what sound quality is. Sound quality refers to the degree of closeness between the actual sound wave and the original waveform. That is, the closer the actual sound wave played back is to the waveform saved in the original audio file, the better the sound quality. Suppose there is an audio file A.wav, and there is an ideal recording device that can record the sound in the air without loss and save it as B.wav. The closer A.wav is to B.wav (in both time domain and frequency domain), the better (for more information, please read the original article on chinaaudio.net: What is sound quality).